1. Field of the invention
The present invention relates to a voltage-controlled oscillation circuit (VCO circuit), and for example, to a RF-CMOS VCO circuit used for a high frequency band.
2. Description of the Related Art
FIG. 13 is a circuit diagram showing a conventional RF-CMOS voltage-controlled oscillation (VCO) circuit 900 used for a high frequency band. Such a conventional RF-CMOS VCO circuit used for a high frequency band, is disclosed in the IEE JOURNAL OF SOLID-STATE CIRCUITS VOL. 32, NO. 5, pp. 736-744, xe2x80x9cA 1.8-GHz Low-Phase Noise CMOS VCO using Optimized Hollow Spiral Inductorsxe2x80x9d, published in May 1997. The conventional RF-CMOS VCO circuit 900 includes a pair of inductors 901a and 901b, a pair of variable capacitance elements 902a and 902b, a pair of MOS transistors M1 and M2 each of which operates as a negative resistance, and a pair of MOS transistor M3 and M4 each of which operates as a current generator.
The variable capacitance elements 902a and 902b are, for example, a pair of pn-connection diodes or a pair of MOS condensers. A capacitance value of the variable capacitance elements 902 is controlled by changing a voltage level, which is applied, to the variable capacitance elements 902a and 902b. 
When an inductance of the inductor 901 is L and a capacitance value of the variable capacitance element 902 is C, an oscillation frequency f is 1/[2 Π{square root over ( )}LC]. When a change of voltage applied to the variable capacitance element 902 is xcex94V, and then a change of the capacitance value C is xcex94C, a ratio xcex94C/xcex94V between the change xcex94V of the voltage applied to the variable capacitance element 902 and the change xcex94C of the capacitance value is determined on the basis of kind of the variable capacitance elements that are used. Then, a change of the oscillation frequency xcex94f=[xe2x88x92f/2C])xc3x97xcex94C. A ratio xcex94f/xcex94V between the change xcex94f of the oscillation frequency and the change xcex94V of the voltage applied to the variable capacitance element 902 is designated a frequency sensitivity xcex94f/xcex94V=[xe2x88x92f/2C]xc3x97[xcex94C/xcex94V]. Specifically, two outputs (oscillation frequency xcex94f=[xe2x88x92f/2C]xc3x97xcex94C and frequency sensitivity xcex94f/xcex94V =[xe2x88x92f/2C]xc3x97[xcex94C/xcex94V] is determined on the basis of two parameters (inductance L and capacitance value C). A center capacitance value of the variable capacitance element 902, or a capacitance value when a tuning voltage is 0 V, can be used as the capacitance value C.
In the conventional RF-CMOS VCO circuit used for a high frequency band, the oscillation frequency f is used around 1 GHz, the inductance L used is around a few nH, and the capacitance value C of the variable capacitance element 902 is around a few pF. The inductor 901 has a series resistance (wiring resistance) which even increases in accordance with an increase of the inductance L. The series resistance of the inductor 901 is larger than the wiring resistance of another connection of the conventional RF-CMOS VCO circuit. Therefore, the series resistance of the inductor 901 deteriorates a quality factor Q of the conventional RF-CMOS VCO circuit.
When the inductor 901 having a smaller series resistance and the variable capacitance element 902 having a larger variable capacitance are used in the conventional RF-CMOS VCO circuit, the conventional RF-CMOS VCO circuit can obtain a larger quality factor Q. However, in such a case, the frequency sensitivity xcex94f/xcex94V=xe2x88x92f/(2C)xc3x97(xcex94C/xcex94V)deteriorates.
The present invention is therefore directed to providing a voltage-controlled oscillation (VCO) circuit which substantially overcome one or more of the problems due to the limitations and disadvantages of the related art.
It is an objective of the invention to provide a voltage-controlled oscillation (VCO) circuit, including a current generator, a variable capacitor having a capacitance value which changes in accordance with a tuning voltage, and which is electrically connected to the current generator in series, an inductor which is electrically connected to the variable capacitor in parallel, a fixed capacitor which is electrically connected to the variable capacitor in parallel, and which has a set capacitance.
According to the present invention, the oscillation frequency, the frequency sensitivity and the quality factor can be set to a desired value, individually. Further, a phase noise can be reduced.